Marathonisi, also known as Turtle Island, is a small uninhabited island located off the southern coast of Zakynthos (also known as Zante), a Greek island in the Ionian Sea. The island is approximately 1.5 km long and 500 meters wide, and is part of the National Marine Park of Zakynthos, which was established to protect the endangered Loggerhead sea turtle (Caretta caretta) and its nesting sites.

Marathonisi has become a popular destination for tourists visiting Zakynthos, particularly for its pristine beaches and clear turquoise waters. The island is also known for its lush vegetation, including olive trees and wildflowers, and is a popular spot for snorkeling and scuba diving.

Visitors can reach Marathonisi by taking a boat tour from the nearby port of Keri, which takes approximately 30 minutes. The island is surrounded by shallow waters, making it easy to swim or wade to shore. There are no permanent structures on the island, and visitors are encouraged to leave no trace of their visit to help preserve the island's natural beauty and protect the sea turtle nesting sites.